The present invention relates, in general to radio frequency (RF) communication systems, and is particularly directed to an RF power amplifier distortion correction mechanism, that employs a swept oscillator to locate and isolate the RF carrier component in the RF power amplifier output, so that distortion energy produced at the output of the amplifier may be detected. Once detected, the distortion energy may be controllably removed by a digital signal processor-controlled distortion cancellation device, such as pre-distortion unit installed in the input path of the RF power amplifier, or a gain/phase adjustment unit installed in the error path of a feed-forward RF amplifier.
The specifications and regulations of the Federal Communications Commission (FCC) mandate that communication service providers comply with very strict bandwidth constraints, including the requirement that the amount of energy spillover outside a licensed channel or band of interest, be sharply attenuated (e.g., on the order of 50 dB). Although such limitations may be readily overcome for traditional forms of modulation, such as FM, they are difficult to achieve using more contemporary, digitally based modulation formats, such as M-ary modulation.
Attenuating sidebands sufficiently to meet industry or regulatory-based standards using such modulation techniques requires very linear signal processing systems and components. Although relatively linear components can be obtained at a reasonable cost for the relatively low bandwidths (baseband) of telephone networks, linearizing components such as power amplifiers at RF frequencies can be prohibitively expensive.
A fundamental difficulty in linearizing an RF power amplifier is the fact that it is an inherently non-linear device, and generates unwanted intermodulation distortion products (IMDS). IMDs manifest themselves as spurious signals in the amplified RF output signal, separate and distinct from the RF input signal. A further manifestation of IMD is spectral regrowth or spreading of a compact spectrum into spectral regions that were not occupied by the RF input signal. This distortion causes the phase-amplitude of the amplified output signal to depart from the phase-amplitude of the input signal, and may be considered as an incidental (and undesired) amplifier-sourced modulation of the RF input signal.
A straightforward way to implement a linear RF power amplifier is to build it as a large, high power device, but operate the amplifier at a only a low power level (namely, at a small percentage of its rated output power), where the RF amplifier""s transfer function is relatively linear. An obvious drawback to this approach is the overkill penaltyxe2x80x94a costly and large sized RF device. Other prior art techniques which overcome this penalty include feedback correction techniques, feedforward correction, and pre-distortion correction.
Feedback correction techniques include polar envelope correction (such as described in U.S. Pat. No. 5,742,201), and Cartesian feedback, where the distortion component at the output of the RF amplifier is used to directly modulate the input signal to the amplifier in real time. Feedback techniques possess the advantage of self-convergence, as do negative feedback techniques in other fields of design. However, systems which employ negative feedback remain stable over a limited bandwidth, which prevents their application in wide-bandwidth environments, such as multi-carrier or W-CDMA. Feedforward and predistortion correction, however, are not limited in this regard.
In the feedforward approach, error (distortion) present in the RF amplifier""s output signal is extracted, amplified to the proper level, and then reinjected with equal amplitude but opposite phase into the output path of the amplifier, so that (ideally) the RF amplifier""s distortion is effectively canceled.
With predistortion correction, a signal is modulated onto the RF input signal path upstream of the RF amplifier. The ideal predistortion signal has a characteristic, which is the inverse of the distortion expected at the output of the high power RF amplifier, so that when subjected to the distorting transfer function of the RF amplifier, it effectively cancels the distortion behavior.
Either predistortion or feedforward may be made adaptive by extracting an error signal component in the output of the RF amplifier and then adjusting the control signal(s), in accordance with the extracted error behavior of the RF amplifier, so as to effectively continuously minimize distortion in the amplifier""s output.
One of the conventional mechanisms for extracting the error signal component is to inject a pilot (tone) signal into the signal flow path through the amplifier and measure the amplifier""s response. A fundamental drawback to the use of a pilot tone is the need for dedicated pilot generation circuitry and the difficulty of placing the pilot tone within the signal bandwidth of the amplifier. Other approaches employ a high intercept receiver to detect low level distortion in the presence of high power carriers, which adds substantial complexity and cost.
In accordance with the present invention, RF power amplifier distortion is accurately measured, even in the presence of multi-frequency input signals, by using a swept local oscillator to tune respective RF input and output receivers. The power detected by the tuned input receiver is compared with a power reference to determine the presence of carrier at the amplifier""s input. Whenever the power detected by the input receiver exceeds the power of the referencexe2x80x94indicating the presence of carrier energy within the tuned receiver""s bandwidthxe2x80x94a similar signal path through the output tuned receiver may be controllably blanked with a high isolation switch. As a result, as the output receiver is swept across the bandwidth of the amplifier output signal, only distortion energy will be detected by the output receiver. The distortion energy detected by the output receiver may be digitized and processed to control pre-distortion correction circuitry upstream of the RF amplifier, or gain/phase adjustment circuitry in the error path of a feedforward error correction loop.
Pursuant to a first, dual (input-output) receiver-based embodiment of the present invention, an adaptive predistortion circuit is installed upstream of an RF power amplifier that has a relatively xe2x80x9clowxe2x80x9d carrier-to-interference distortion ratio (C/I) output signal. By relatively a low C/I ratio output signal is meant one in which the RF carrier level is effectively indistinguishable from that of intermodulation products, such as for the case of mixed modulation multicarrier signals and multicarrier signals having different power levels. (In contrast, a relatively xe2x80x98highxe2x80x99 C/I output signal, such as that produced at the output of a highly linear RF amplifier with equal power carriers, is one in which the level of the RF carrier is readily distinguishable from that of the IMDs.)
In the first, predistortion embodiment of the invention, the RF input signal to be amplified is coupled through a directional coupler to an input mixer and an IF bandpass filter used as part of a swept input receiver, which detects the presence of carrier energy at the input to the RF amplifier. Whenever the carrier energy detected by the input receiver exceeds a predefined threshold, a controllably swept output receiver coupled through a directional coupler to the output of the RF amplifier is blanked by a threshold detector. The output of the threshold detector is monitored by a digital signal processor (DSP) controller to keep track of where (in the swept spectrum) carrier energy is located.
A common sweep frequency for each of the input and output receivers is derived from the same local oscillator, that is controlled by a digital sweep-control signal generated by the DSP. The output of the swept oscillator is split and fed to respective mixers of the input and output receivers. The IF output of the input mixer is filtered by a slightly wider bandpass filter and coupled to a carrier energy detector, whose output is monitored by a threshold detector. The output of the threshold detector is coupled to a blanking detector input of the DSP and to control ports of isolation switches in the output receiver.
During controlled variation (e.g., sweep) of the drive frequency for the input and output receivers, as long as the output of the carrier energy detector does not exceed a prescribed threshold associated with an RF carrier signal, the signal flow path through the output receiver is considered to be representative of amplifier distortion, and is therefore detected as an error signal by the DSP. In response to this error signal the DSP adaptively adjusts the parameters of a predistortion unit in order to compensate for the distortion.
However, if the detected carrier energy exceeds the prescribed carrier-associated threshold, the output of the threshold detector changes state, providing both a blanking signal to the DSP and a control signal to blank (interrupt) the signal flow path through the output receiver. In this manner, the DSP""s adjustment of the parameters of the predistortion unit will remain independent of the presence of an RF carrier. Moreover, such carrier-based selective blanking of the distortion measurement receiver circuitry prevents saturation of the output receiver""s IF amplifier, and allows the use of lower third order intercept (IP3) components.
In accordance with a second, predistortion embodiment of the invention for use with an RF power amplifier having a high C/I ratio, the circuit architecture of the controllably blanked distortion energy measurement subsection is simplified. In particular, the input receiver mixer is eliminated, leaving only the output receiver mixer, which downconverts the output of the RF power amplifier. To allow for carrier threshold-based blanking, the downconverted output receiver""s mixer is split into two paths: one to a wider band carrierxe2x80x94threshold detector, the other to a narrower band distortion detector, via isolation switches.
As in the first embodiment, the output of the carrier detector is compared with a threshold during the frequency sweep of the local oscillator. Whenever the detector output exceeds the thresholdxe2x80x94indicating the presence of carrier energy within the carrier detector""s bandwidthxe2x80x94a blanking signal is generated, so as to interrupt the signal path to the distortion detector.
In addition to applying the invention to measure distortion for adjusting the parameters of a predistortion unit upstream of the RF amplifier, the invention may be employed in an RF power amplifier distortion measurement and correction scheme, in which a DSP-controlled adaptive predistortion adjustment circuit is installed in a feed-forward cancellation amplifier path downstream of the RF amplifier. Again, either a low or a high C/I ratio version of the controllably blanked distortion energy measurement subsection described above may be employed, depending upon the amplifier""s characteristics.
Pursuant to a third embodiment of the invention, a DSP-controlled, adaptive gain/phase adjustment circuit is installed in the error path of a feed-forward amplifier, which utilizes a relatively low C/I ratio main RF power amplifier. The RF input port to the-main RF power amplifier is coupled to a first RF signal loop that includes an upstream gain/phase adjustment circuit, such as a vector modulator. The RF input port is further fed through a directional coupler to a second RF signal flow path via a delay line to a first port of an RF carrier cancellation combiner of a feed-forward error extraction and reinjection loop. A portion of the amplified signal output of the RF amplifier is extracted and coupled to a second port of the carrier cancellation combiner. The carrier cancellation combiner serves to cancel a time-aligned RF carrier component in the second RF signal flow path from the output of the RF amplifier and provides an RF error signal representative of the distortion or IMDs.
The RF error signal produced by the RF cancellation combiner is coupled to a DSP-controlled gain/phase adjustment circuit for the feed-forward error correction and reinjection loop. The output of this gain/phase adjustment circuit is amplified in a feed-forward RF error amplifier and reinjected into the output path of the main RF amplifier. In order to monitor and adaptively control, the gain and phase of the feed-forward error path, its associated control processor is supplied with amplifier distortion signals by way of a controllably blanked distortion energy measurement subsection, configured and operating in the same manner as the first embodiment, described above.
In accordance with a fourth embodiment of the invention for a relatively high C/I ratio amplifier, the dual receiver-containing controllably blanked distortion energy measurement subsection of the low C/I amplifier of the third embodiment is replaced by the reduced complexity single receiver-based, controllably blanked distortion energy measurement subsection of the second embodiment.